Here is an outline of my progress so far:
I have finished writing the code for loading texture and altitude maps, and I have created several tools for changing the view and applying textures to the map. The code for selecting map cells is complete. Cells can be selected one at a time or multiple at a time while holding down "Control". It is also possible to "drag-select" with the mouse. I made it so that if any cells are selected, the paint tool will automatically fill them with whatever texture I pick. I made it so right-click always acts like the select tool, so I won't have to keep switching back and forth between tools. I added in a visible wireframe that I can turn on or off from the Options toolbar. This helps to distinguish one cell from the next. The "Save" button in the options toolbar works nicely. The "Load" function, however, isn't finished. I've also added in buttons for "Select by cell", "Select by map", and "Free select". The later two will be implemented later after I add in the ability to load more than one adjacent maps at a time.